In this chapter, we discuss the physics of any two junctions formed between two crystalline solids both in equilibrium and nonequilibrium. In general there are many types of junctions that can be formed between two different crystalline materials. Specifically, the junctions we are most concerned with are p–n homojunctions, p–n or n–n heterojunctions, metal–semiconductor junctions, and metal-insulator–semiconductor (MIS) junctions. The p–n homojunction consists of n- and p-type layers made from the same material type, a common silicon p–n junction diode, for example. A heterojunction is formed from two dissimilar material types that are often doped differently as well. For example, a common heterojunction of great use in modern semiconductor devices is that formed from n-type AlGaAs on either intrinsic GaAs or p-type GaAs.

In addition to semiconductor–semiconductor junctions, metal–semiconductor and MIS junctions can be formed as well. The two most important types of metal-semiconductor junctions are Schottky barriers, which have diodelike, rectifying current-voltage characteristics, or ohmic contacts, which have linear current-voltage characteristics.

Knowledge of the equilibrium and the nonequilibrium properties of these junction types, along with the earlier topics covered in this book, will provide us with sufficient background to study advanced semiconductor devices in the next chapters. First we consider the equilibrium properties of each junction type. Next we consider the nonequilibrium current-flow processes in each junction. Our method is to treat these different junction types, when possible, by using a unified approach.